Learning topic: Assist after surgery
Use of autoclave
At normal atmospheric pressure, (sea level) the highest temperature
produced by steam and boiling water is 100 ºC. Destruction of heat resistant
micro-organisms will not be achieved at this temperature.
However, if the pressure around the boiling water is increased 15 psi
(pounds per square inch) above normal atmospheric pressure, the
temperature of the boiling water and steam will be increased to 121 oC. At
this temperature, all micro-organisms including heat resistant spores will be
destroyed.
The use of steam under pressure:
Autoclaving
The autoclaves used in vet practices function in a similar way to the
pressure cookers used in the home, with a central sterilising chamber
surrounded by a steam jacket.
The large units in hospitals are usually connected to a central boiler of
steam, whereas the bench-top varieties incorporate an electrically driven
boiler.
A surgical pack neatly wrapped in green linen is placed on top of a trolley. There
are two bundles of folded linen on the second layer of the trolley. On the right is a
sterilising machine.
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Why remove air from the sterilisation chamber?
Steam is lighter than air, so to ensure that all items in the sterilising chamber
come in contact with the steam, the air must be evacuated.
The air which is at a lower temperature than the steam, is displaced
downwards by the steam, so with a mixture of air and steam present,
sterilising time would have to be increased or the pressure increased to
ensure an adequate temperature was reached.
When does sterilisation begin?
Sterilisation begins as the steam enters the sterilising chamber and
condenses on the colder surfaces. This condensation into water produces
heat which penetrates the packaged items from the outside through to the
centre.
Moisture increases the penetrability of the heat, so it is important that the
items are arranged to facilitate this.
After the specified time of sterilisation has elapsed, steam is allowed to
escape from the chamber, which is followed by a fall in temperature.
The autoclave door should not be opened until the pressure gauge has
dropped to zero and the temperature gauge is below 100 ºC.
Most modern autoclaves incorporate a drying cycle at the end of
sterilisation.
Some autoclaves also have safety mechanisms on the door to prevent the
door from being opened until the pressure and temperature are at the correct
levels.
The table below shows that the higher the temperature and pressure of the
steam, the shorter the length of time required for sterilisation to occur.
Temperature
(degrees
Centigrade)
Pressure
121
15
15
126
20
10
134
30
3.5
Time (minutes)
(psi >atmos.)
Types of autoclaves
There are several types of autoclaves.
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Learning topic: Assist after surgery
Vertical pressure cooker
This cooker operates similarly to a household pressure cooker. It is
commonly used in practice.
The vertical pressure cooker has an air vent at the top which is manually
closed after all the air has been evacuated. This is a possible disadvantage as
some air may remain trapped under the steam and therefore cause a lower
temperature to be reached in this area.
As this is a manually operated steriliser, human error can influence the
effectiveness of sterilisation.
There is also no automatic drying cycle, so items may be slightly damp at
the end of the cycle. Ensure packs are fully dried before storing.
Horizontal or downward displacement autoclave
This type of autoclave is electrically operated and can be fully or semiautomatic. The bench-top varieties are common in practice.
These are usually designed for loose instrument sterilisation but can
accommodate small packs. The steam outlet is at the top and air outlet at the
bottom. This is a more efficient operation as air is driven out by downward
displacement.
The drying cycle is inadequate, so ensure proper drying of packs before
storage.
Steam pulsings systems
Steam is actively pulsed into the chamber to reach a given pressure and then
the chamber is vented to a minimal pressure before the next pulse. The cycle
time is generally shorter than downward displacement systems.
The steam pulsings system is cheaper than pre-vacuum sterilisers.
Vacuum assisted or pre-vacuum autoclave
Air is evacuated rapidly from the sterilising chamber at the beginning of the
cycle creating a vacuum, so steam penetration and sterilising occurs very
rapidly. A second vacuum cycle withdraws moisture after sterilisation,
facilitating drying of the load. Often, this autoclave is connected to a central
boiler of steam.
They have a large capacity and are fully automated and failsafe.
Modern, computerised autoclaves will abort the cycle if there is a
malfunction.
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Features of the autoclave
Whatever the type of autoclave, each one should have the following
features:

a source of saturated steam under pressure, either from a steam line or
water placed in the autoclave prior to operation

a pressure gauge indicating the pressure within the chamber

a safety valve to prevent explosion should the pressure become too high

method of removing air

shut off valve to prevent the escape of steam once the air has been
removed

a temperature gauge

a valve to release steam at the end of sterilisation

a door seal

a timer

safety locking mechanism for the door.
Monitoring the effectiveness of
sterilisation
Processing an item in an autoclave does not necessarily guarantee its
sterility.
All items to be sterilised should be clean and dry before being packed.
Damp items will encourage moisture inside the surgical pack at the end of
the sterilising process. This contaminates the item as moisture promotes
bacterial growth.
Effective sterilisation will only occur and be maintained if all the following
steps are carried out:

thorough cleaning and drying of equipment is done before sterilisation

correct wrapping and packaging of equipment

correct loading of items into the autoclave chamber

correct sterilisation cycle is selected in autoclave

post-sterilising drying of the load occurs

appropriate handling and storage of sterilised items to maintain sterility.
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© NSW DET 2007
Learning topic: Assist after surgery
Storage of sterilised packs
When packs are not to be used immediately after autoclaving they should be
stored in a dry place, preferably on a shelf in a closed cupboard. Sterilised
packs that are stored open to the atmosphere have a shorter shelf life than
those that are stored in a cupboard.
Packs should be stored only when they are dry, as the presence of moisture
enhances the growth of micro-organisms.
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© NSW DET 2007